1. Field of the Invention
The invention described and claimed herein is generally related to positive displacement rotary gas compressors. More particularly, the present invention is related to a class of positive displacement rotary gas compressors commonly known as Roots pumps.
2. Description of the Related Art
The class of positive displacement gas compressors known as Roots compressors, or Roots blowers, has been known and used in industry for over a hundred years. It is well recognized in industry that for certain applications Roots compressors offer a number of advantages over other types of gas pumps and compressors, for example conventional piston-and-cylinder reciprocating pumps, fan-type blowers and turbine pumps. Among these advantages are simplicity, ruggedness, trouble-free operation, and high volumetric capacity. There are no valves, pistons or other reciprocating mechanical parts, and there are no rubbing mechanical parts. Additionally, there is little or no backflow, or leakage, even when the Roots compressor is not operating. A typical application of a Roots compressor is the transfer or evacuation of large amounts of toxic or corrosive gas, where it is important to rapidly pump large amounts of gas with little or no backflow. In this type of application reciprocating pumps are relatively inefficient, and fan-type blowers and turbine pumps cannot provide a seal against backflow.
Roots compressors most commonly include two lobed impellers (sometimes also called rotors) which intermesh with one another and rotate in opposite directions in synchronization within a housing. The impellers operate to sweep a gas through the housing from an intake manifold at one end of the housing to an output manifold at the opposite end of the housing. Although commercially available Roots compressors most commonly include impellers having only two lobes, Roots compressors have also been designed to include impellers having three, four and possibly even more lobes. Two-lobed impellers however are the most common because they are simpler and because the volumetric efficiency of a Roots compressor is inversely proportional to the proportion of the compressor chamber that is occupied by the impellers, and two-lobed impellers generally occupy the least volume.
While Roots compressors are extraordinarily efficient for the purpose of rapidly moving large volumes of gas where there is little or no pressure differential, they have heretofore been of limited application for the purpose of pumping a gas against a pressure differential. This limitation has been due to heating effects which attend such pumping. As a gas is swept through a Roots compressor from a region of relatively low pressure to a region of relatively higher pressure, it is compressed and heated. Such compression is essentially adiabatic, such that the temperature of the gas rises exponentially with increasing pressure ratios. The increase in the temperature of the gas leads to heating of the impellers, the housing and other mechanical parts of the pump. This in turn can lead to thermal distortion, expansion and friction. At pressure ratios of greater than about two to one (2:1) such effects become a significant problem. Overheating of the compressor can result in lockup or other mechanical failure of the seals, impellers and other compressor components.
The heating problem is not uniform throughout the compressor. The compressor housing, for example, can be cooled by a number of conventional methods, such as the use of integral double-walled water jackets, heat radiating fins and the like. The greatest heating problem however lies with the impellers, as there is no practical way to directly cool the impellers. Overheating of the impellers leads to their expansion and eventual binding against the housing, possibly causing extensive damage to the compressor. Overheating of the Roots compressor has thus been one of the most significant limitations on the use of Roots compressors for pumping gas against pressure differentials, and for this reason commercially available Roots compressors are typically limited to pressure ratios of less than about four to one (4:1).
Perhaps the most simple and straightforward method of avoiding the adverse effects of overheating is to increase the clearances between the impellers and the housing, thereby allowing the impellers to expand somewhat on heating without rubbing and locking up against the housing. This however necessarily leads to increased leakage and backflow, and thereby degrades the volumetric efficiency of the compressor. For this reason this approach has not generally been considered a satsifactory solution to the overheating problem.
A substantial advance in the art was the development of recirculation cycles to effect a moderate reduction in the heating of Roots compressors. In a recirculating Roots compressor, a portion of the output gas, which is compressed and heated, is cooled, external to the compressor, and subsequently recirculated back into the compressor. This effectively reduces the rise in temperature of the gas within the compressor, thereby mitigating the heating problems described above.
U.S. Pat. No. 2,489,887 to Houghton, for example, discloses the concept of cooling the impellers of a Roots compressor by introducing recirculated gas of a lower temperature into the intake gas to reduce heating of the impellers. The teachings of the Houghton patent are discussed in greater detail below, in comparison with the present invention.
In prior art recirculating Roots compressors, such as the compressor disclosed in Houghton, the flow of recirculating gas is either interrupted each time a rotor lobe passes the recirculation entry port, or is halted and possibly reversed as a displacement cavity is simultaneously opened to both a recirculation port and a discharge port. This results in a loss of momentum and flow of the recirculation fluid, reducing the efficiency of the recirculation fluid in cooling the compressor. This problem, which is inherent in previously known Roots compressors, is overcome in the present invention, as will be made apparent by the descriptions set forth below.
Accordingly, it is the object and purpose of the present invention to provide an improved positive displacement rotary gas compressor.
It is also an object and purpose of the present invention to provide a positive displacement rotary compressor capable of sustained operation without overheating at high pressure ratios.
It is also an object and purpose of the present invention to provide a positive displacement rotary compressor having an improved gas recirculation means for reducing heating of the compressor.
It is a further object and purpose of the present invention to provide a positive displacement rotary compressor characterized by having a continuous, uninterrupted flow of cooled recirculation gas flowing from the output of the compressor back into the compressor housing.
It is also an object of the present invention to provide a positive displacement rotary compressor that produces less heat inside the compressor and is thus capable of operating at higher pressure ratios than have been previously sustainable.